Pitch to pitch online gray balance calibration with dynamic highlight and shadow controls

ABSTRACT

An automatic gray balance control system to produce TRCs for all primary colors in a reproduction device and for each pitch of a photoreceptor system by printing target patches for each pitch, measuring the output colors, and automatically readjusting the tone reproduction curves until a satisfactory level of accuracy is obtained as compared to the theoretical desired output. The system produces pitch-based gray balanced TRCs that are updated frequently for each pitch, with different TRCs for different pitches, to ensure consistency in output from pitch to pitch as well as from page to page on a given pitch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.09/566,291, filed on 5 May 2000 and entitled, “Online Calibration Systemfor a Dynamically Varying Color Marking Device;” U.S. patent applicationSer. No. 09/862,247, filed on 22 May 2001, now U.S. Pat. No. 6,621,576,issued on 16 Sep. 2003, and entitled, “Color Imager Bar BasedSpectrophotometer for Color Printer Color Control System;” U.S. patentapplication Ser. No. 09/862,945, filed on 22 May 2001, now U.S. Pat. No.6,633,382, issued on 14 Oct. 2003, and entitled, “Angular, Azimuthal andDisplacement Insensitive Spectrophotometer for Color Printer ColorControl Systems;” U.S. patent application Ser. No. 09/863,042, filed on22 May 2001, now U.S. Pat. No. 6,556,300, issued on 29 Apr. 2003, andentitled, “Color Imager Bar Based Spectrophotometer PhotodetectorOptical Orientation;” U.S. patent application Ser. No. 09/949,475, filedon 10 Sep. 2001, now U.S. Pat. No. 6,639,669, issued on 28 Oct. 2003,and entitled, “Diagnostics for Color Printer On-line SpectrophotometerControl System;” U.S. patent application Ser. No. 10/248,387, filed on15 Jan. 2003, and entitled, “Systems and Methods for Obtaining a SpatialColor Profile and Calibrating a Marking System;” U.S. patent applicationSer. No. 10/342,873, filed on 15 Jan. 2003, and entitled, “IterativePrinter Control and Color Balancing System and Method Using a HighQuantization Resolution Halftone Array to Achieve Improved Image Qualitywith Reduced Processing Overhead;” U.S. patent application Ser. No.11/070,681, filed on 2 Mar. 2005, and entitled, “Gray Balance for aPrinting System of Multiple Marking Engines;” U.S. patent applicationSer. No. 11/097,727, filed on 31 Mar. 2005, and entitled, “Online GrayBalance Method with Dynamic Highlight and Shadow Controls;” thedisclosures of which are incorporated by reference in their entirety.

BACKGROUND AND SUMMARY

Embodiments are generally related to printing methods and systems.Embodiments are also related to developing tone reproduction curves thatfacilitate consistent and accurate printing from pitch to pitch on aphotoreceptor and/or an intermediate transfer belt and/or other markingelement.

Embodiments refer to printing as the art of producing a pattern, such astext and images, on a substrate, such as paper or transparent plastic. Amarking engine performs the actual printing by depositing ink, toner,dye, or similar patterning materials on the substrate. For brevity, theword “ink” will be used to represent the full range of patterningmaterials. In the past, the pattern was introduced to the marking enginein the form of a printing plate or a light lens. Modernly, digital dataare commonly used to specify the pattern. The pattern can be a data filestored in a storage device and/or transmitted to the printer via anetwork, radio transmission, infrared radio transmission, and the like.

A popular marking engine today is the xerographic marking engine used inmany digital copiers and printers. In such a marking engine, aphotoreceptor whose electrostatic charge varies in response to light isplaced between an ink supply and the substrate. In xerographic systems,the ink is typically toner. A laser or bank of light emitting diodes isused to expose the photoreceptor to light to form an image of thepattern to be printed on the photoreceptor. In the simplest,monochromatic xerographic engines, toner is applied to the image tocreate a toner image on the photoreceptor, which toner image is thenfused onto the substrate. In more complex systems, additional colors oftoner are applied.

Color systems include Image On Image (IOI) systems and tandem systems.In an 101 system, such as that shown schematically in FIG. 1, the engine10 includes plural primary colors 11 which deposit their inks on thephotoreceptor 13, which includes multiple pitches 14. The singlephotoreceptor 13, such as a belt, receives the first toner image in afirst color, which remains on the photoreceptor 13 while a second tonerimage is created in a second color atop the first image, the first andsecond toner images remain on the photoreceptor while a third tonerimage is created in a third color atop the first and second images, etcetera. Once all of the toner images have been placed on thephotoreceptor 13, they are transferred to the substrate, typicallypaper, and fused to the substrate.

In an embodiment of tandem system architecture, such as that shown inFIG. 2, the marking engine 20 includes multiple primary colors 21 whichfirst deposit their inks on respective photoreceptors 22, typicallydrums, to form toner images, which are then deposited on theintermediate transfer belt (ITB) 23, which includes multiple pitches 24.Each toner image is transferred onto the ITB before the next toner imageis formed. Like the IOI system, the toner images are fused once all fora given pitch have been deposited on the ITB.

In a variant of the tandem system shown in FIG. 2, each ink station caninclude an additional drum between the photoreceptor and the ITB, anintermediate drum, that accepts the toner image from the photoreceptordrum and deposits it on the ITB. The inclusion of the intermediate drumreduces the likelihood of toner of another color getting into a givenink source due to electrostatic interactions between the toner image onthe ITB and the photoreceptor drum. Each of the printing architecturesfound in the marketplace has advantages, but all suffer from colorreproduction problems.

In color science, color spaces are used to describe colors. For example,the Pantone colors are a color space commonly used by graphic artists toidentify different colors. Another important color space defined by theCIE is known as L*a*b*, where L*, a*, and b* specify color coordinates.One of the most important properties of L*a*b* is that it is deviceindependent. In other words, a L*a*b* color will theoretically be thesame regardless of when or how it is produced and by what particulardevice it is produced. However, because of the nature of ink andmarking, particularly color ink, placing a mixture of ink on substratethat should theoretically produce a particular L*a*b* color does notnecessarily produce that particular color. One of the commonly used waysin which the difference between the desired color and the printed one isquantified by its Euclidian distance in color space. If L*₀a*₀b*₀ andL*₁a*₁b*₁ are the L*a*b* of the desired color and of the printed color,then the difference, ΔE, is defined by the following equationΔE=((L ₁ *−L ₀*)²+(a ₁ *−a ₀*)+(b ₁ *−b ₀*))^(1/2)  (1)It should be appreciated that many other color difference equations arealso in use, and not disclosed in this application, which considerperceptual aspects of human visual system. Equation (1) above is thus anexample color difference expression.

Part of the reason for the discrepancy between desired and obtainedcolors is that a different color space, CMYK, is commonly used inprinting. The letters CMYK refer to the cyan, magenta, yellow, and blackinks that color printers typically use and are primary colors in suchsystems. Mixing these inks produces the other colors that a markingengine can print.

The problem with CMYK is that it is not device independent for variousreasons. One such a reason is that the pigments of inks are notnaturally balanced, and their equal combination does not produce aneutral gray. Another reason is that different inks from differentsources mix differently on different substrates. For example, in onesituation, a certain combination of cyan, magenta, and yellow ink willproduce a particular shade of gray. In another situation, thecombination could produce a greenish gray. Of course, the color spacewill be different in printers using other or additional primary colors.For example, some printers add Orange and Violet, creating asix-dimensional color space, but the problem of color variance remains.

Interestingly, it has been found that compensating for color variancethroughout the color gamut of the color printer can be achieved byadjusting the ink mixture to produce gray level balance. This can beperformed by printing one or more test patches based on particularrequested gray levels, analyzing the output with a spectrophotometer,and generating a tone reproduction curve (CRC). The TRC is then used toalter the theoretical combination of ink to produce more accurate colorwith an actual combination.

An example of this method is seen in U.S. patent application Ser. No.11/097,727, filed 31 Mar. 2005 and entitled, “Online Gray Balance Methodwith Dynamic Highlight and Shadow Controls,” incorporated by referenceabove. TRCs are used to map an input value to an output value as seen,for example, in FIG. 9, to adjust ink application levels.

To show how TRCs can be employed to adjust ink application levels, a TRCfor one of the color separations is shown in FIG. 9, albeit not toscale. The input axis 401 and the output axis 402 both have saturationvalues ranging from 0 to 255, with 0 indicating no saturation (no ink onthe substrate and 255 indicating complete saturation (as much ink aspossible on the substrate). Saturation values between 0 and 255 indicateintermediate amounts of ink are deposited. Without a TRC, a request for100 yellow based on a desired color representation results in acorresponding amount of ink. As described above, the inclusion of 100yellow may not yield the desired color, and with a TRC, a request for100 yellow can be mapped to a different amount of ink that will producethe desired color. In FIG. 9, when 100 units of ink are input 403, theTRC maps the input to an actual value of 107 output 404. The TRC of FIG.9 thus maps a request for 100 units of ink into a request for 107 unitsof ink to produce the desired color.

When using cyan, magenta, yellow, and black inks to produce a processgray. TRCs can be used to more accurately produce a desired gray. If,for example, one desires a process gray of 128 cyan, 128 magenta, 128yellow, and 0 black, but the marking engine used must employ 131 cyan,127 magenta, and 130 yellow, and 0 black to achieve the desired result,TRCs can adjust the requested amounts so that the marking enginedeposits 131 cyan, 127 magenta, 130 yellow, and 0 black, yielding thedesired process gray. Preferably, a different TRC is used for each inkthat a marking engine uses so that a CMYK marking engine will have fourTRCs. TRCs can have different ranges of saturation values, such as 0 to1, 0 to 100, or 0-255. Regardless of the input range and output range,all TRCs are used to adjust the amount of ink deposited by mapping aninput value to an output value.

In the '727 application, a TRC is normally produced by an algorithm thatfits a curve to a series of knots, which can be determined fromcalibration data. Printing a calibration patch pattern yields a targetpatch pattern. The desired reflectances of the calibration patches andthe measured reflectances of target patches can be used as calibrationdata. The series of knots can also include a highlight knot and a shadowknots so that the TRC functions better in the highlight and shadowregions.

More specifically, the '727 application discloses a system and methodfor producing TRCs that work well over all saturation values, includinghighlights and shadows, by supplying data to produce better TRCs forhighlights and shadows. A storage device stores a calibration patchpattern and the calibration patch pattern includes at least twocalibration patches. A marking engine can produce a target patch patternby printing the calibration patch pattern.

The system produces a target patch pattern by using a marking engine toprint a calibration patch pattern on a substrate. The calibration patchpattern includes at least two calibration patches. Each calibrationpatch is developable and has a desired reflectance. When the targetpatch pattern is produced, each calibration patch is printed as a targetpatch. The system obtains target reflectances by measuring targetpatches that are in the target patch pattern. At least two targetreflectances can be obtained because the target patch pattern has atleast two target patches. The system then determines a target highlightvalue from data that includes an input highlight value, the targetreflectances, and the desired reflectances. The calibration datapreferably includes at least one target saturation and at least onemaximum desired saturation. Target saturation relates to the amount ofink that is deposited on a substrate. The target saturation can be themaximum amount of a particular ink that the marking engine can depositon the substrate. The particular ink can be black or a primary colorsuch as cyan, magenta, or yellow. Calibration data can be used toproduce a tone reproduction curve. The method can be enhanced byallowing a user to select a target saturation for any of the inks,including cyan, magenta, yellow, or black, that a marking engine uses. Acalibration patch based on the user selected saturation can be printedto produce a target patch whose target reflectance is obtained bymeasuring the target patch. The target reflectance can then be includedin calibration data used to produce a tone reproduction curve. Obtainingat least two target reflectances for the patch, a processor uses thetarget reflectances and input highlight value to produce a targethighlight value and a tone reproduction curve which is stored on astorage device.

The '727 application disclosure, however, does not deal with thevariations seen from pitch to pitch in multiple pitch systems. Becausethe magnitude of pitch signature changes with time due to variousreasons (e.g., by developer aging, IBT aging, differential belt wear,etc.), embodiments disclosed herein include a calibration and controlmethodology for achieving high quality and consistent color balancedprinting for printers with periodic pitch-to-pitch variations.Preferably, calibration methods for single pitches can be employed, suchas the method referred to above. Embodiments contemplate having graybalanced TRCs and updating them frequently for each pitch, thus havingdifferent TRCs for different pitches. Embodiments use customized TRCsfor each pitch during the course of printing to obtain consistencybetween pages printed on different pitches. Additionally, embodimentscan obtain a customized gray balanced CMYK TRCs for each pitch usingcontrol based iterative gray balance methods using a reduced patch setwith as few as twenty-two patches, which is easy to schedule to graybalance the print engine on a per pitch basis. Typically, thecalibration job is performed as a separate job, then a print job ormultiple print jobs are performed before another calibration job isperformed. However, embodiments contemplate such calibration during runtime.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, in which like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which are incorporated in and form a part of the specification,further illustrate the present invention and, together with thebackground of the invention, brief summary of the invention, anddetailed description of the invention, serve to explain the principlesof the present invention.

FIG. 1 schematically illustrates an Image On Image (IOI) printing engineshowing multiple pitches on the photoreceptor.

FIG. 2 schematically illustrates a tandem printing engine showingmultiple pitches on the intermediate transfer belt (ITB).

FIG. 3 schematically illustrates a marking engine undergoing calibrationby producing a TRC according to embodiments.

FIG. 4 schematically illustrates a possible target patch patternaccording to embodiments.

FIG. 5 shows some possible relationships between patch patterns, colorspaces, and measurements according to embodiments.

FIG. 6 shows an example of a L* coordinate plot of 50% gray patches withrespect to pitch number, printed for 120 belt revolutions, according toembodiments.

FIG. 7 shows the corresponding a* plot for the 50% gray patches shown inFIG. 6 according to embodiments.

FIG. 8 show the corresponding b* plot for the same 50% gray patchesshown in FIG. 6 according to embodiments.

FIG. 9 schematically illustrates a TRC for a color separation accordingto embodiments.

FIG. 10 schematically illustrates a calibration method according toembodiments

FIG. 11 schematically illustrates a variation of a calibration method ofembodiments.

DESCRIPTION

The particular values and configurations discussed in these non-limitingexamples can be varied and are cited merely to illustrate embodiments.They are not intended to limit the scope of the invention.

As described above, when equal amounts of cyan, magenta and yellow areprinted on white paper, a well-balanced printer should produce a neutralprocess gray of the same amount. However, the system will usually notproduce that gray due to various limitations on color pigments of theprimary colors used by and the internal processes of the print engine.To overcome this effect, gray balanced TRCs obtained by iterativemethods, such as those described above and disclosed U.S. applicationSer. Nos. 09/566,291, 11/070,681, and 11/097,727, incorporated byreference above, can be employed to apply the right amount of cyan,magenta and yellow proportions for all contone values depending on thestate of the materials and the print engine. This approach can beextended to produce gray balanced TRCs for spatial uniformitycorrections as disclosed, for example, in U.S. Patent Applications Nos.10/248,387 and 10/342,873, incorporated by reference above. However,none of these methods have discussed an approach for reducing grayvariations on a pitch-to-pitch basis.

As disclosed above, there are many factors that contribute topitch-to-pitch variations, but photoreceptor/ITB variations are amongthe root causes for consistency errors in images. Substantialdifferences in gray have been observed, for example ΔE>3, when the samegray images are printed on different pitches in a photoreceptor belt.This difference is even larger for saturated colors.

Yet it has been found that if the pitch-to-pitch variation of grayimages is reduced, the saturated colors also show improvements.Embodiments thus contemplate a method to generate gray balanced TRCsthat are customized to each pitch during the process of building theTRCs. This approach can be implemented in the digital front end (DFE) ofa marking engine or in the image path of a marking engine, such as in acolor rendition module. Embodiments are particularly advantageous wheninline sensors can be used.

The calibration and control methodology of embodiments achieves highquality and consistent color balanced printing for printers withperiodic pitch-to-pitch variations. The factors discussed above causethe magnitude of pitch signature to change over time, and the graybalanced TRCs, preferably updated frequently for each pitch, withdifferent TRCs for each pitch, can balance such variations. Usingcustomized TRCs for each pitch during the course of printing yieldsconsistency between pages printed on different pitches. Additionally,embodiments can obtain a customized gray balanced CMYK TRC for eachpitch using control based iterative gray balance methods with a reducedpatch set. For example, embodiments including as few as twenty-twopatches in a calibration patch pattern have been successful. As aresult, it is easy to schedule the TRCs to gray balance the print engineon a per pitch basis.

FIG. 3 illustrates a marking engine 102 undergoing calibration accordingto an exemplary method of generating a tone reproduction curve that canbe used in embodiments. This exemplary method is based on that disclosedin U.S. patent application Ser. No. 11/097,727, incorporated byreference above. A storage device 101 stores a calibration patch pattern111 in the form of data. The calibration patch pattern 111 includes anumber of calibration patches and every calibration patch has a desiredreflectance. As such, the storage device 101 also stores desiredreflectances 109. A reflectance can specify any color, including blackand shades of gray. The marking engine 102 accepts the calibration patchpattern and prints a target patch pattern 103. The target patch pattern103 includes target patches 104. Every target patch 104 is associatedwith a calibration patch because every target patch 104 results from theprinting of a calibration patch. Examples of the particular calibrationof embodiments are described below.

A reflectance measuring device 105, such as the reflectance measuringdevice disclosed in U.S. Pat. No. 6,384,918 to Hubble et al., whichissued on May 7, 2002 and which is incorporated herein by reference,measures the target patches 104 to produce target reflectances 110. Atarget reflectance generally is the reflectance measurement that thereflectance measuring device 105 obtains from a target patch 103. Thetarget reflectances 110 and the desired reflectances 109 are used by aprocessor 106 to produce a tone reproduction curve 107 which can then bestored on a storage device 108.

FIG. 4 illustrates one possible target patch pattern that can be used inembodiments. While the pattern shown includes 22 patches divided intotwo portions over two pages, this need not be the particular number ofpatches or the particular configuration, and embodiments contemplateplacing the entire pattern on a single page as well as using differentnos. of patches. The pattern preferably includes a series of black inkproduced gray patches 202, a series of primary color patches 203, and aseries of process gray patches at various saturation levels 212. Ifrequired, such as due to limitations of sensors used to evaluate thepatches, the pattern of FIG. 4 can be broken into a first page 201 and asecond page 211.

The series of black patches 202 includes patches with various levels ofblank ink or toner saturation across the range achievable by theprinter. For example, the black patches 202 can include patches between5% and 90% saturation. The paper outside of and between the patches canbe measured to find the reflectance of unpatterned substrate areas. Theblack patches 202 are formed using only black ink. The series of primarycolor patches 203 preferably includes a saturated patch of each primarycolor, such as cyan ink, magenta ink, and yellow ink. As seen in FIG. 4,embodiments can include eight black patches 202 and one primary colorpatch 203 for each primary color.

The series of process gray patches 212 in embodiments are printed usingthe CMY inks to produce gray patches. One patch 213 is preferably notmarked with ink toner and can be used, for example, to characterize thesubstrate color. The CMY gray patches 212 are used in conjunction withthe black patches 202 to provide tone reproduction curves for cyan,magenta, yellow and black ink separations, as disclosed, for example, inU.S. patent application Ser. No. 11/097,727, incorporated by referenceabove. As seen in FIG. 4, the method employs ten CMY/process graypatches 212 and one unmarked patch 213, but other numbers of suchpatches could be employed.

The target patch pattern of embodiments can be divided into twoportions, particularly when U.S. Letter (8.5″×11″) paper or the like isused, as seen in FIG. 4 and mentioned above. A first page 201 includesthe black patches 202 and primary color patches 203; and a second page211 includes the process/CMY gray patches 212 and the unmarked patch213. This arrangement is beneficial when a sensor used to analyze thevarious patches is limited, such as in the number of such patches it candetect at a given time. In the case of the arrangement shown in FIG. 4,the target patch pattern of embodiments and the overall method ofembodiments can be used in a machine using a sensor limited to elevenpatches at a time for U.S. Letter (8.5″×11″) paper. Other arrangementsof the target patch pattern can be used as appropriate for various typesof sensors and sizes of paper. For example, the entire pattern could beformed on a single page.

FIG. 5 shows some possible relationships between patch patterns, colorspaces, and measurements. L*a*b* pattern 301 can be used to specify thedesired output from a marking engine. Mapping between color spaces 302produces a CMYK pattern 303 from the L*a*b* pattern. The mapping can bedifferent for different situations because L*a*b* is invariant and CMYKis not. Mapping for a specific marking engine 305 involves using tonereduction curves (TRCs) 304 to adjust the CYMK pattern 303 to produce aCMYK pattern ready for printing 306. The pattern can then be printed onthe substrate. Usually, nothing more is done once the printed pattern isproduced.

More, however, can be accomplished. For example, the printed pattern canbe measured 308 for quality control or calibration purposes. A measuringdevice, such as the in-line spectrophotometer disclosed in U.S. Pat. No.6,384,918, incorporated by reference in its entirety, can measure thereflectance of some areas of the printed pattern to produce an L*a*b*target reflectance 309. Comparing the L*a*b* pattern 301 to the L*a*b*target reflectance 309 can reveal the differences between the markingengine's desired output and its actual output. In quality controlscenarios, small enough differences can indicate acceptable quality. Incalibration scenarios, the differences can be used to adjust the TRCs.Proper adjustment of the TRCs can minimize the differences.

In calibration scenarios, the L*a*b* pattern 301 can be a calibrationpatch pattern. When a calibration patch pattern is printed, the printedpattern is a target patch pattern such as that shown in FIG. 4. TRCs 304can be used during calibration, but it is sometimes more convenient notto use them. When no TRC is used, the CMYK pattern 303 and the patternready for printing 306 are equivalent. A target patch pattern ismeasured by determining the reflectance of individual patches in thepattern. Furthermore, target patch patterns can have patches of manydifferent colors. For example, a target patch pattern can have cyan,magenta, yellow and black patches. It can have gray patches producedwith black ink. It can have gray patches produced by printing acombination of cyan, magenta, and yellow inks. In general, a targetpatch pattern can have patches of any color, shade, or saturation thatis obtainable with the inks and the marking engine.

FIG. 6 shows an example of a L* coordinate plot of 50% gray patches withrespect to pitch number, printed for 120 belt revolutions in aprototypical printer. FIGS. 7 and 8 show the corresponding a*, and b*plots for the same 50% gray patches. Tables 1 and 2 show mean L*, a* andb* values and the corresponding ΔE numbers with respect to each pitchusing the mean L*a*b* values.

TABLE 1 Mean L*, a*, b* values for 120 belt revolutions shown withrespect to pitch Pitch# Mean L* Mean a* Mean b* 1 52.98 12.9 0.34 250.99 14.87 0.08 3 52.45 13.55 0.27 4 52.41 13.63 0.56 5 52.75 13.610.62 6 52.82 13.22 0.65 7 53.21 13.03 0.71 8 53.01 13.15 0.57 9 52.4614.39 0.52 10 50.66 15.17 0.01

TABLE 2 ΔE between various pitches with respect to a given pitch (zerofor ideal engine) ΔE with respect to Pitch Numbers Pitch# 1 2 3 4 5 6 78 9 10 1 0 2.81 0.84 0.95 0.79 0.47 0.46 0.34 1.59 3.26 2 2.81 0 1.981.95 2.24 2.54 2.96 2.71 1.61 0.45 3 0.84 1.98 0 0.3 0.47 0.62 1.02 0.750.88 2.43 4 0.95 1.95 0.3 0 0.35 0.59 1.01 0.77 0.76 2.4 5 0.79 2.240.47 0.35 0 0.4 0.75 0.53 0.84 2.68 6 0.47 2.54 0.62 0.59 0.4 0 0.440.22 1.23 2.98 7 0.46 2.96 1.02 1.01 0.75 0.44 0 0.27 1.57 3.41 8 0.342.71 0.75 0.77 0.53 0.22 0.27 0 1.36 3.16 9 1.59 1.61 0.88 0.76 0.841.23 1.57 1.36 0 2.03 10 3.26 0.45 2.43 2.4 2.68 2.98 3.41 3.16 2.03 0

For an ideal print engine, the variation of ΔE in Table 2 should benegligible. Instead, a maximum of 3.4 ΔE has been observed. To minimizepitch-based ΔE variation, embodiments modify the digital contone imageor the binary contone image using different gray balanced TRCs one TRCfor each pitch. Generating these TRCs for each separation on apitch-to-pitch basis and manipulating the digital value of CMYK imagesdifferently for each pitch according to embodiments can reduce oreliminate the problem.

In particular, embodiments focus on building gray balanced TRCs usingmeasurements from a limited set of mixed color patches, such as thosedescribed above in connection with FIG. 4. A method for doing so isschematically illustrated in FIGS. 10 and 11. Gray balanced TRCs can begenerated accurately according to embodiments using, for example,approximately twenty-two mixed CMY gray patches and K patches in similarfashion to that employed by some prior art methods, such as thatdisclosed in Mestha et al., “Gray Balance Control Loop for Digital ColorPrinting Systems,” Proceedings of 21^(st) International Conference onDigital Printing Technologies, NIP21, pp. 499-505 (2005), which isincorporated by reference in its entirety. However, unlike the prior artsolutions, embodiments give TRCs for a coarse correction of thepitch-to-pitch variation, thereby overcoming low frequency variations inΔE with respect to pitch like those shown in FIG. 6 and Table 2. Sinceembodiments use relatively few gray and black patches, it is easier toschedule the patches for measurements on a particular pitch in order toconstruct TRCs more frequently, thus reducing time dependent drifts inperformance.

An example of gray balance patches used to obtain gray balanced TRCs inembodiments, using the target patch pattern of, for example, FIG. 4,employs a range of CMY gray patches 212, fully saturated primary colorpatches 203, and a range of black patches 202. For example, the CMY graypatches 212 can have values of 0, 5, 8, 10, 17, 25, 32, 40, 50, 60, and74 percent saturation, the CMY solid patches 203 of 100C, 100M, and100Y, and K patches 202 of 7.5, 10, 15, 25, 40, 55, 75, and 90 percentsaturation. These particular values have been advantageous inembodiments, but other values can be employed. The intervals betweenarea coverage values are preferably irregular, and more patches arepreferred to be in the highlight range of from 0 to about 20 percentsaturation. These 22 patches are distributed across the neutral axis(CMY neutral and K patches) and include three saturated cyan, magenta,and yellow patches. FIG. 4 shows an example of such a gray balancetarget as it would appear printed on 8.5″×11″ paper.

To obtain pitch-based gray balanced TRCs, the target patches ofembodiments, particularly those seen in FIG. 4, should be scheduledcarefully. An example of a method of doing so according to embodimentsis shown in FIGS. 10 and 11. The method can comprise producing a targetpatch pattern for each pitch 801, measuring each target patch pattern toobtain measured values 802, determining a TRC for each pitch based onthe measured values 803, applying each TRC to its respective pitch 804,and repeating until a difference between desired and actual values isless than a threshold 805. Producing a target patch pattern 801 caninclude producing at least one process gray patch, at least oneblack-based gray patch, and at least one patch of each primary color901. Reproduction apparatus of the type in which embodiments can beemployed typically include an inline spectrophotometer that is capableof reading one test patch every 3 ms. In printing machines using such aspectrophotometer and able to print about 100 ppm, embodiments requiretwo 8.5″×11″ pages 201, 211 for each pitch to acquire the necessaryinformation for TRC construction. Thus, as seen in FIG. 11, producing atleast one process gray patch, etc., 901 can include producing 11 processgray patches 902, producing 8 black-based gray patches 903, andproducing 1 color patch with maximum coverage values for each primarycolor 904. Producing 901 can also include producing the process graypatches on a first page for each pitch 905 and producing the black-basedand color patches on a second page for each pitch 906. Producing 11process gray patches 902 can include producing patches with areacoverage values of about 0, 5, 8, 10, 17, 25, 32, 40, 50, 60, and 74,but, as mentioned above, other values can be used. Similarly, producing8 black based patches can include producing patches with area coveragevalues of about 7.5, 10, 15, 25, 40, 55, 75, and 90, though other valuescan be used.

To accomplish this method, embodiments thus first schedule printing onefirst target page 201 for each pitch, which is 10 pages of the graybalance target first page for a 10-pitch photoreceptor, each with 11patches. Subsequently, embodiments schedule one second target page 211for each pitch, which is another 10 pages of the gray balance targetsecond page for a 10-pitch photoreceptor, each with the additional 11patches. The gray balance targets are grouped per pitch to construct theTRC so that the first target page printed on the first pitch is groupedwith the second target page printed on the first pitch during the secondrevolution of the belt, the first target page printed on the secondpitch is grouped with the second target page printed on the second pitchduring the second revolution of the belt, and so on. For a 10-pitchphotoreceptor, a total of 20 pages would be required to complete themeasurements required for extracting all of the pitch-based TRCs. Thegeneralization for other media size, e.g., 11″×17″ paper, isstraightforward.

To obtain TRCs for a given pitch, embodiments use the measured L*a*b* ofthe target patches corresponding to a given pitch to computecorresponding L*a*b* values relative to paper. It should be noted that,in some other implementations of this technology, absolute L*a*b* valuesare used. The CMY neutral patch values are then used to obtain CMY TRCs.In the 22-patch example above, 21 such L*a*b* values are computed, andthe 10 L*a*b* values from the CMY (neutral) patches are used to obtainCMY TRCs using the method outlined in U.S. Pat. No. 6,621,576, U.S.patent application Ser. No. 11/097,727, and in Mestha et al., “GrayBalance Control Loop for Digital Color Printing Systems,” Proceedings of21^(st) International Conference on Digital Printing Technologies,NIP21, pp. 499-504 (2005), incorporated by reference above. The valuesfor cyan, magenta and yellow are used to find the ΔE vs. white pointcurve, in which ΔE scales linearly with the density and area coverage ofthe input patches. The L* values for the black patches are then used tocompute black TRCs. This process is repeated for all other patchescorresponding to the remaining belt pitch to obtain the remainingpitch-based TRCs and is iterated until converging to a reasonably goodgray. Generally, one to two iterations are found sufficient to reachconvergence to acceptable gray accuracy.

An alternate approach to building the average pitch-based gray balanceTRCs in embodiments comprises scheduling each page containing 11 patcheswith CMY gray patches of the same digital value for the wholephotoreceptor. For a 10-pitch belt, 10 pages of similar patches (e.g.,gray 25) would be scheduled, and a total of 220 pages would be requiredto complete the measurements required for remaining gray levels forextracting all of the pitch-based TRCs. Once the target pattern pagesare generated, the 11 TRCs per pitch can be derived as described above.The average of the 11 TRCs for each pitch will then yield thepitch-based TRCs.

Thus, as seen generally in FIGS. 10 and 11, embodiments use an automaticgray balance control system to produce TRCs for all of the primarycolors in a reproduction device by printing patches at different pitch,measuring the output colors, and automatically readjusting the tonereproduction curves until a satisfactory level of accuracy is obtainedas compared to the theoretical desired output. Pitch-based gray balancedTRCs that are updated frequently for each pitch, with different TRCs fordifferent pitches, is new. The prior art methods do not cover the use ofpitch-based TRCs in images. The method of embodiments fills that gap andcan enhance the application of consistent color in electrophotographicreproduction and printing.

As with many computer-implemented methods, embodiments can beimplemented in the context of modules. In the computer programming arts,a module can be typically implemented as a collection of routines anddata structures that performs particular tasks or implements aparticular abstract data type. Modules generally can be composed of twoparts. First, a software module may list the constants, data types,variable, routines and the like that that can be accessed by othermodules or routines. Second, a software module can be configured as animplementation, which can be private (i.e., accessible perhaps only tothe module), and that contains the source code that actually implementsthe routines or subroutines upon which the module is based. Thus, forexample, the term module, as utilized herein generally refers tosoftware modules or implementations thereof. Such modules can beutilized separately or together to form a program product that can beimplemented through signal-bearing media, including transmission mediaand recordable media.

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims

1. A periodic pitch-to-pitch gray balance printer calibration andcontrol method comprising: producing a target gray patch pattern foreach of a plurality of pitches on a substrate comprising; producing atleast one process gray patch; producing at least one black-based graypatch; measuring each target patch pattern to obtain measured values foreach patch in each target pattern, wherein measured values are obtainedby measuring the reflectance, color shade, and saturation of individualcyan, magenta, yellow, and black in each patch; determining a tonereproduction curve for each pitch based on the measured values byprinting particles of different pitch, measuring the output colors, andautomatically readjusting the tone reproduction curves until asatisfactory level of accuracy is obtained based on a comparison with atheoretical desired output; applying the tone reproduction curves totheir respective pitches to normalize and optimize printer output; andrepeating the steps of producing, measuring, determining, and applyinguntil a difference between desired and actual values is less than apredetermined threshold, thereby calibrating the printer.
 2. The methodof claim 1 wherein producing a target patch pattern further comprises:producing at least one patch of each primary color used by the printer.3. The method of claim 2 wherein producing at least one process graypatch includes producing a plurality of process gray patches with anarea coverage value in the range of from about 0 to about 100 percent.4. The method of claim 2 wherein producing at least one black-based graypatch includes producing a plurality of patches with area coveragevalues in the range of from about 0 to about
 90. 5. The method of claim2 wherein producing at least one patch for each primary color used bythe printer includes producing patches of each primary color with areacoverage values of about
 100. 6. The method of claim 1 wherein measuringincludes employing a color sensor to measure L*a*b* values of thepatches of each target patch pattern.
 7. The method of claim 6 furthercomprising scheduling one page per pitch including process gray patches,and one pager per pitch including base color and black-based patches. 8.The method of claim 1 wherein determining a tone reproduction curvecomprises: obtaining calibration data comprising at least one targetsaturation and at least one maximum desired saturation; using thecalibration data to produce a tone reproduction curve, thereby settingsaid tone reproduction curve for use in printing saturated areas.
 9. Themethod of claim 8 wherein one of the at least one target saturation is aprimary color's maximum possible saturation.
 10. The method of claim 8wherein one of the at least one target saturation is black's maximumpossible saturation.
 11. The method of claim 8 wherein the at least onetarget saturation is a user selected saturation.
 12. The method of claim11 further comprising producing a target patch by printing a calibrationpatch based on the user selected saturation, measuring a targetreflectance of the target patch, and wherein the calibration datafurther comprises the target reflectance.
 13. A system comprising: astorage device adapted to store at least one target gray patch patterncomprising at least two target patches for each pitch of a multiplepitch element; a marking engine that includes a multiple pitch elementwith a plurality of pitches, the marking engine marking a substratebased on the at least one target gray patch pattern to produce a targetgray patch pattern for each pitch; a color measuring device obtains atarget reflectance from each target gray patch of the target patchpattern; a processor that determines at least one tone reproductioncurve from calibration data comprising the target reflectances; a secondstorage device adapted to store said at least one tone reproductioncurve for each pitch of the photoreceptor.
 14. The system of claim 13wherein said calibration data further comprises at least one targetsaturation and at least one maximum desired saturation.
 15. The systemof claim 14 wherein the at least one target saturation comprises atleast one primary color's maximum possible saturation.
 16. The system ofclaim 14 wherein one of the at least one target saturation is black'smaximum possible saturation.
 17. The system of claim 14 wherein the atleast one target saturation comprises at least one user selectedsaturation.
 18. The system of claim 17 wherein at least one of the atleast one target patch is based on the at least one user selectedsaturation and wherein the calibration data further comprises the atleast one maximum desired saturation and the at least one user selectedsaturation.
 19. The system of claim 13 wherein said at least twocalibration patches comprise at least two calibration patches printedwith black.
 20. The system of claim 13 wherein said at least twocalibration patches comprise at least two calibration patches printedwith at least one primary color.